Many types of communications are possible over data networks, including electronic mail, web browsing, file downloads, electronic commerce transactions, text chat, and so forth. Networks include private networks, such as local area networks (LANs) or wide area networks (WANs), and public networks, such as the Internet.
With the increased capacity and reliability of packet-based data networks, voice communications (including telephone calls, video conferencing, and so forth) over data networks have become possible. Voice communications over packet-based data networks are unlike voice communications in a conventional public-switched telephone network (PSTN), which provides users with dedicated, end-to-end circuit connections (e.g., one or more time slots of a given channel) for the duration of each call. In a circuit-switched network, speech and control data are routed over the dedicated circuit connection during the call session. In contrast, communications over packet-based data networks, such as IP (Internet Protocol) networks, are performed using packets or datagrams that are sent in bursts from a source to one or more destination nodes. In a packet-based network, a dedicated channel portion is not provided; instead, the communications network is shared, with routing of packets performed based on addresses carried in the packets.
The types of equipment that may be used to perform telephony communications over packet-based data networks include computers that have voice processing components, including a speaker and a microphone. Such computers also include audio coder/decoders (CODECs), implemented in hardware and/or software, that convert between audio and digital data. The telephony application running in such a computer is often referred to as a soft phone application, and the computer itself may be referred to as a “soft phone.” One example soft phone application is the i2050 product from Nortel Networks Limited.
Another type of terminal that is capable of telephony communications over a packet-based data network is a network telephone, such as the i2004 telephone set from Nortel. A network telephone may include a network interface and various layers (including network and transport layers) to enable communication of voice data over the data network.
In conventional circuit-switched telephony networks, telephones are coupled to a switch, such as a private exchange system (e.g., a PBX or private branch exchange system, a key telephone system, and so forth). A signaling channel between the switch and the telephone carries control signaling such as an alert message (to indicate an incoming call), commands to update the telephone display, an off-hook indication, an on-hook indication, an on-hold indication, signals carrying button or key presses of the telephone, and so forth. Depending on which physical port of the switch the telephone is connected to, a terminal number (TN) is assigned to the telephone by the switch.
The switch is able to establish a circuit-switched call connection between a first telephone (connected to the switch) and a second telephone (which may or may not be connected to the switch. If the second telephone is coupled to another switch, then signaling and data are exchanged between the switches over a trunk. The trunk can be an Integrated Services Digital Network (ISDN) link, such as a basic rate interface (BRI) or primary rate interface (PRI).
With the advent of packet-based network telephony (e.g., IP telephony), terminals (e.g., network telephones or soft phones) are no longer connected directly to a switch. Instead, the terminals are coupled over a LAN, WAN, Internet, or other packet-based network to a packet network-enabled switch system. In some arrangements, the network-enabled switch system includes both a network interface (to the packet-based network) and a circuit network interface (to a circuit-switched line such as a time-division multiplexing or TDM line). On a TDM line, plural time slots are defined on a channel, with one or more time slots assigned to communications between the telephone and the switch.
In a call between two network terminals, the network-enabled switch converts voice data of a first network terminal from a packet format to a circuit-switched data format (e.g., TDM format). The network-enabled switch then converts the voice data back into packet format for communication to the second network terminal.
For a call between a first network terminal connected to a first network-enabled switch and a second network terminal connected to a second network-enabled switch, multiple conversions between TDM and packet formats may have to be performed. For example, voice data from the first network terminal is converted from a packet format to TDM format and communicated over a trunk (e.g., a circuit-switched trunk) from the first switch to the second switch. In the second switch, the voice data in TDM format is reconverted back to packet format and communicated to the second network terminal.
When performing conversions between TDM and packet format, degradation of voice data may occur. Degradation may result from compression and decompression of the voice data, algorithmic delay, and effects of jitter buffers. The degradation is made even worse if the trunk between the first and second switches is a packet-based trunk. In this scenario, the voice data from the first network terminal is converted from a packet format to a TDM format by the first switch, followed by conversion from the TDM format to a packet format in the first switch for communication over the packet-based trunk. Next, the second switch converts the voice data received over the packet-based trunk from the packet format to TDM format, followed by conversion from the TDM format back to the packet format for communication to the second network terminal.